Satellite signal deformation can be caused by a satellite fault or a natural satellite bias. This can deform the signal's correlation peak which will result in different range measurements for different receiver designs (filtering and correlation processes). Hence, signal deformation introduces errors that cannot be canceled by differential GPS operation when the ground and airborne receivers are of different designs. Signal deformation monitors (SDM) are designed to detect such a deformation and discontinue use of a satellite's measurement data before the satellite fault impact increases the integrity risk to the airborne user to an unacceptable level. Some implementations of Signal Deformation Monitors utilize a Code Carrier Divergence (CCD) monitor. Such monitors are effective in detecting some instantaneous changes, particularly for signal deformation faults where the SDM detection may exceed time-to-alert requirements. Fault detection performance of the signal deformation and CCD monitors may be established by comparing residual range errors to time-varying maximum allowable range errors.
One problem, which is particularly prevalent near equatorial regions, is that a false SDM alarm can be triggered even when no real fault is present. When this occurs, it is frequently caused by ionospheric activity. As such, the problem of false SDM alerts is frequently encountered near equatorial regions where the ionosphere is significantly more active.
For the reasons stated above and for other reasons stated below, which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for systems and methods for differentiating actual satellite fault events from false SDM alarms caused by ionospheric activity.